1. Field of the Invention
The present invention relates to a method and apparatus for cleaning a photomask to be used as a master in the photolithography step in the process for the production of semiconductor integrated circuit device (LSI), where the photomask requires an extremely clean surface.
2. Description of the related Art
A photomask serves as a master when an integrated circuit pattern is transferred to the surface of a wafer by a transferring apparatus in the photolithograpy step in the process for the production of semiconductor integrated circuit device. If the pattern formed on the surface of the photomask has defects or foreign objects present thereon in an amount exceeding the critical resolution, they can be transferred to the wafer as a part of pattern.
Therefore, any defects or foreign objects beyond the critical resolution are not allowed to occur on the surface of the photomask.
With the development of integration density and precision of integrated circuit, the maximum allowable size of defects or foreign objects has been reduced to 0.5 .mu.m.
The conventional method of cleaning a photomask has employed a cleaning method based on RCA cleaning (cleaning with a mixture of an acid such as sulfuric acid and hydrogen peroxide or a mixture of an alkaline agent such as aqueous ammonia and hydrogen peroxide), which is also widely used for cleaning wafers.
A flow chart of the cleaning process is shown in FIG. 11.
A conventional photomask cleaning step will be described in connection with FIG. 11.
Firstly, in the step 1, cleaning is effected with a hot mixture of sulfuric acid and hydrogen peroxide to decompose organic objects such as resist and residual solvent present on the surface of a photomask and remove metallic impurities.
This step provides the surface of the photomask with an improved wettability that enhances the efficiency of the subsequent cleaning steps.
Subsequently, in the step 2, the photomask is rinsed with hot pure water to remove residual agents such as sulfuric acid from the surface thereof.
Subsequently, in the step 3, the photomask is dipped in and cleaned with a heated mixture of ammonia and hydrogen peroxide for the purpose of removing attached foreign objects.
During this step, an ultrasonic wave such as a megasonic wave may be applied to the dipping tank to remove foreign objects more effectively.
This step, too, needs to be followed by rinsing with pure water as shown in the step 4.
Finally, the photomask which has been thus rinsed is dried in the step 5.
The step 3 may involve cleaning with pure water alone or mixed with a detergent under the application of an ultrasonic wave such as megasonic wave,rather than with the mixture of ammonia and hydrogen peroxide.
The dip cleaning method as mentioned above allows the simultaneous dipping of a plurality of photomasks to enhance the cleaning efficiency but has a disadvantage that extremely contaminated photomasks contaminate relatively clean photomasks.
In order to surmount this desadvantage, as a process involving the use of a cleaning fluid on a throwaway basis for every sheet of photomask there has been employed a cleaning process (spin process) which comprises spraying onto the surface of a photomask which is horizontally rotated a cleaning fluid such as chemical agent and pure water from a fixed or swinging nozzle so that the cleaning fluid is used on a throwaway basis every sheet of photomask.
The spin cleaning process may involve a mechanical cleaning such as high pressure jet pure water rinsing and megasonic pure water rinsing to effectively remove foreign objects.
The conventional cleaning process as shown in FIG. 11 is disadvantageous in that the treatment with sulfuric acid/hydrogen peroxide (step 1) is sometimes followed by insufficient cleaning possibly leaving some amount of sulfuric acid behind that causes fogging on the surface of the photomask.
Fogging on the surface of the photomask reduces the transmittance of the photomask on the unpatterned area (area which transmits light on the wafer), causing a variation of the dimension of a resist patternwise formed on the wafer. Particularly in the case of half tone photomask, since an exposure light penetrates slightly semitransparent pattern such as MoSiON film, fogging on the surface of the photomask reduces the transmittance of the photomask on the semitransparent pattern as well as the unpatterned area. As a result, resolution of the resist pattern formed on the wafer is lowered. This can break the wiring in integrated circuit (LSI), deteriorating the properties of LSI itself.
In the conventional cleaning process, the treatment with sulfuric acid/hydrogen peroxide (step 1) is followed by rinsing with a large amount of pure water or hot pure water (step 2) to prevent the remaining of sulfuric acid. However, this rinsing step consumes a large amount of pure water and considerable electric energy required to heat pure water.
In the dipping process for the treatment with ammonia/hydrogen peroxide for the purpose of removing foreign objects, a plurality of photomasks are treated with the same batch of cleaning fluid. Therefore, the cleaning fluid must be renewed so frequently to prevent the deterioration or contamination thereof, adding to the required amount thereof.
Further, if the cleaning efficiency (yield in cleaning) is poor, the number of cleaning times required per sheet of photomask increases. Therefore, the required amount of cleaning fluid, pure water and energy such as electric power increases.
In recent years, as shown in FIGS. 12A-12D a phase shift photomask which can improve a resist resolution on the wafer by partly changing the phase transmitted thereby has been developed and put into practical use. FIG. 12A represents Levenson type of phase shift photomask, FIG. 12B represents Sub Pattern type of phase shift photomask, FIG. 12C represents edge empathizing type of phase shift photomask, and FIG. 12D represents half tone type of phase shift photomask. Numeral 300 is a quartz substrate, 301 is a light shielding pattern such as a CrON film and 302 is a phase shifter such as a MoSiON film. 303 is an etching stopper.
An MoSiON film is a material used for light-shielding layer (semi transparent layer) in a halftone photomask, which is one of these phase shift photomasks. However, when subjected to conventional cleaning with alkali solution, e.g., involving dipping in ammonia/hydrogen peroxide, it shows a drastic change in transmittance or phase angle caused by increasing a surface roughness and thickness reduction. Particularly in the case of halftone photomask, since resolution of the resist pattern formed on the wafer depends on the change in transmittance or phase angle, such a change in transmittance or phase angle causes very serious problem. Therefore, it is not possible to maintain a desired product quality when shipped.
Accordingly, in practice, the cleaning with ammonia/hydrogen peroxide effective for the removal of foreign objects (step 3) cannot be effected for MoSiON film. Thus, MoSiON film has been cleaned with pure water alone or a detergent, causing the remaining of foreign objects.